perf(qt): virtualize discovery and warm playback

cx

crates/qbz-qt/qml/theme/QbzScrollBar.qml

@@ -14,9 +14,11 @@ Item {
     visible: maxScroll > 0
 
     readonly property real maxScroll: Math.max(0, target.contentHeight - target.height)
-    readonly property real thumbH: target.contentHeight > 0
-        ? Math.max(48, target.height / target.contentHeight * height)
-        : height
+    // ListView/GridView origins are not guaranteed to stay at zero when their
+    // model or variable-size delegates change. Use the documented Flickable
+    // coordinate (`contentY - originY`) for both drawing and seeking.
+    readonly property real scrollY: Math.max(0, Math.min(maxScroll, target.contentY - target.originY))
+    readonly property real thumbH: target.contentHeight > 0 ? Math.max(48, target.height / target.contentHeight * height) : height
     readonly property real travel: Math.max(0, height - thumbH)
     // Shown while scrolling, hovering, or dragging (auto-hide).
     property bool scrollActive: false
@@ -25,8 +27,8 @@ Item {
     Connections {
         target: root.target
         function onContentYChanged() {
-            root.scrollActive = true
-            hideTimer.restart()
+            root.scrollActive = true;
+            hideTimer.restart();
         }
     }
     Timer {
@@ -35,6 +37,13 @@ Item {
         onTriggered: root.scrollActive = false
     }
 
+    // One shared input seam for every page that uses the house scrollbar:
+    // physical-wheel acceleration is Qt's native process policy (main.rs),
+    // while this observer supplies only a missing touchpad tail.
+    QbzKineticScroll {
+        target: root.target
+    }
+
     // Track.
     Rectangle {
         width: 6
@@ -43,19 +52,31 @@ Item {
         radius: 3
         color: "#12ffffff"
         opacity: root.shown ? 1.0 : 0.0
-        Behavior on opacity { NumberAnimation { duration: 160 } }
+        Behavior on opacity {
+            NumberAnimation {
+                duration: 160
+            }
+        }
     }
     // Thumb.
     Rectangle {
         width: barArea.containsMouse || barArea.pressed ? 10 : 8
         x: Math.round((parent.width - width) / 2)
         height: root.thumbH
-        y: root.maxScroll > 0 ? (root.target.contentY / root.maxScroll) * root.travel : 0
+        y: root.maxScroll > 0 ? (root.scrollY / root.maxScroll) * root.travel : 0
         radius: 5
         color: barArea.containsMouse || barArea.pressed ? "#77ffffff" : "#44ffffff"
         opacity: root.shown ? 1.0 : 0.0
-        Behavior on width { NumberAnimation { duration: 100 } }
-        Behavior on opacity { NumberAnimation { duration: 160 } }
+        Behavior on width {
+            NumberAnimation {
+                duration: 100
+            }
+        }
+        Behavior on opacity {
+            NumberAnimation {
+                duration: 160
+            }
+        }
     }
     // Drag / click-to-position over the whole gutter.
     MouseArea {
@@ -63,12 +84,20 @@ Item {
         anchors.fill: parent
         hoverEnabled: true
         cursorShape: Qt.PointingHandCursor
-        onPressed: position(mouseY)
-        onPositionChanged: if (pressed) position(mouseY)
+        onPressed: {
+            // Public Flickable semantics already cancel on a contentY write;
+            // make the takeover explicit so scrollbar drag remains exact even
+            // if the first pressed position equals the current one.
+            root.target.cancelFlick();
+            position(mouseY);
+        }
+        onPositionChanged: if (pressed)
+            position(mouseY)
         function position(my) {
-            if (root.travel <= 0) return
-            var frac = Math.min(1, Math.max(0, (my - root.thumbH / 2) / root.travel))
-            root.target.contentY = frac * root.maxScroll
+            if (root.travel <= 0)
+                return;
+            var frac = Math.min(1, Math.max(0, (my - root.thumbH / 2) / root.travel));
+            root.target.contentY = root.target.originY + frac * root.maxScroll;
         }
     }
 }